In semiconductor production, spectroreflectometry is used as a non-destructive analyzing method of thin layer systems. An incident infrared radiation beam reflects off a sample, and the intensity of the reflected radiation is analyzed to determine properties of the sample. The incident radiation includes multiple frequency components or is monochromatic with a time-varying frequency. The reflected radiation is analyzed at a plurality of measuring frequencies such that, for example, a reflectance spectrum may be obtained that represents the frequency dependence of the intensity of the reflected radiation.
By analyzing the obtained reflectance spectrum, the thickness of each thin layer in a multiple layer system covering a semiconductor wafer can be determined through model-based algorithms. The model-based algorithms typically use a multiparameter analysis routine to extract the layer parameters. The analysis routine is a fitting method that matches the measured reflectance spectrum with a calculated reflectance spectrum that is obtained by calculating the respective values for a model having equivalent model parameters such as film thickness, refractive index and graded transition-profile thickness. The analysis varies the model parameters until the measured and the calculated reflectance spectrum have the best match.
Further, Fourier-transform infrared (FTIR) reflectance-spectroscopy methods have been developed as metrology tools for characterizing layer systems on a semiconductor wafer. A Fourier-transform infrared apparatus includes a scanning Michelson interferometer, which allows the simultaneous measurement of multiple wavelengths. A beam splitter separates an initial radiation beam into two beams. The first beam has a fixed path length, while the path length of the second beam is periodically varied. The two beams are then recombined such that interference occurs between the beams according to their optical path difference. In this way, an interferogram is obtained that plots the respective radiation intensity against the mirror position, which is related to the optical path difference. Then a Fourier transformation of the interferogram is performed to obtain the reflectance spectrum, which is then analyzed according to various model-based analyzing methods.
For patterned layer systems having a 3D structure, the model-based fitting algorithms become more complicated. The layer parameters and the simulated 3D structure obtained from the model-based fitting algorithm do not always match well with that of the actual layer system. For example, the model-based fitting algorithms often render insufficient results for samples having a 3D structure with high aspect ratio trenches.